GNSS reception using distributed time synchronization

ABSTRACT

A GNSS receiver communicates with any connectivity device, such as a WiFi device that is, in turn, in communication with a wired network having access to the DTI timing. Such connectivity devices may set their timing and frame synchronization to the DTI and thus serve as Geopositioning beacons, thereby enabling the GNSS receiver to accurately determine its position. The GNSS receiver may also use the DTI timing supplied by such a network to perform relatively long integration time so as to achieve substantially improved sensitivity that is necessary for indoor Geopositioning applications. Furthermore, the GNSS data, such as satellite orbital information, may also be propagated by such devices at high speed. By providing this data to the GNSS receivers via such connectivity devices in a rapid fashion, the GNSS receivers are enabled to receive the transmitted data associated with the satellite without waiting for the GNSS transmission from the satellites.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S.application Ser. No. 12/980,202, filed Dec. 28, 2010, now U.S. Pat. No.8,497,802, which claims benefit under 35 USC 119(e) of U.S. ProvisionalAppln. No. 61/290,449, filed Dec. 28, 2009, entitled “GNSS ReceptionUsing Distributed Time Synchronization”, the contents of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

GNSS receiver systems can increase their sensitivity by integrating thereceived GNSS signals over a longer period of time, so long as the timebase used within the receiver is sufficiently stable over that timeperiod. The stability of the crystal oscillator used in low-costcommercially-available consumer-grade GNSS receiver devices restrictsthe coherent integration time to below 10 seconds. Consumer GNSS devicesare often used in mobile environments, resulting in timing changes dueto their motion.

A typical GNSS receiver system may not have up-to-date satelliteinformation and thus would need to perform an extensive search offrequency and code offsets for each satellite in order to synchronizeits time to the GNSS time and achieve lock. FIG. 1 shows the space thatis searched by a conventional GNSS system, such as a GPS.

When a GNSS receiver initially locks to a satellite it needs to scanfrequency offsets and code offsets to account for the drift in thereceiver's time base, which is often provided by a low-cost crystaloscillator. This search must be repeated if the GNSS receiver stopstracking signals from the GNSS satellite system which may happen for anumber of reasons. The coherent integration time used by consumer-gradeGNSS receivers is limited by a variety of factors including stability ofthe timebase at the receiver, and the prohibitive cost, power and sizeof a timebase which would enable longer integration times. If a GNSSreceiver loses lock for some period of time (such as when it isindoors), its time base will quickly drift away from the GNSS timereference. Such drifts prevent the receiver from acquiring the GNSSsignals or performing coherent integration over longer periods of timeduring acquisition.

BRIEF SUMMARY OF THE INVENTION

A method of locating the position of a wireless receiver, in accordancewith one embodiment of the present invention includes, in part,receiving from a wired network timing information that is substantiallysynchronous with the GNSS clock, transmitting the timing informationwirelessly to the wireless receiver, and locating the position of thewireless receiver using the transmitted timing information. In oneembodiment, the wireless receiver is a GNSS receiver. In anotherembodiment, the wireless receiver is a connectivity device conforming toa wireless standard, such as 802.11a, 802.11b, 802.11g, 802.11n,Bluetooth, WiMax, Zigbee, UWB, 60 GHz, and the like.

A method of locating the position of a device, in accordance with oneembodiment of the present invention includes, in part, receiving from awired network a clock signal that is substantially synchronous with theGNSS clock, transmitting the received clock signal wirelessly, receivingthe transmitted clock signal by a first wireless transceiver,transmitting a first frame from the first wireless transceiversubstantially synchronously with the GNSS clock, receiving thetransmitted first frame by the device, and attempting to identify theposition of the device, in part, using the information received from thefirst frame. In one embodiment, the position of the device is determinedusing, in part, the information received from the first frame togetherwith information representative of the distance between device and asecond wireless transceiver.

In accordance with one embodiment of the present invention, the wirednetwork is a DOCSIS-compliant network supplying DTI timing informationrepresented by the clock signal. In one embodiment, the wired network isfurther adapted to receive ephemeris information associated with GNSSsatellites. In one embodiment, the device is a GNSS receiver.

In one embodiment, the first frame includes Geolocation informationrepresentative of the position of the first wireless transceiver. In oneembodiment, the Geolocation information includes time stamprepresentative of the transmission time of the first frame. In oneembodiment, the method further includes, in part, receiving at least oneGNSS signal from at least one GNSS satellite by the GNSS receiver, andattempting to identify the position of the GNSS receiver usinginformation received from the first frame and the received GNSS signal.

In one embodiment, the method further includes in part, receiving by thesecond wireless transceiver, a second frame transmitted substantiallysynchronously with the GNSS time by a third wireless transceiver, andattempting to identify the position of the second wireless transceiverusing information received from the first frame, second frame, and theGNSS signal. In one embodiment, the method further includes, in part,transmitting the ephemeris information wirelessly to the first wirelesstransceiver; and transmitting the ephemeris information wirelessly fromthe first wireless transceiver to the GNSS receiver. In one embodiment,the method further includes transmitting the clock signal wirelesslyfrom the first wireless transceiver to the GNSS receiver; and enablingthe GNSS receiver to use the clock signal to increase its integrationtime. In one embodiment, the wireless transceiver is a connectivitydevice conforming to a wireless standard, such as 802.11a, 802.11b,802.11g, 802.11n, Bluetooth, WiMax, Zigbee, UWB, 60 GHz, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the space that is searched by a conventional GNSS system,as known in the prior art.

FIG. 2 shows a cable head end supplying the DTI timing signal to anumber of customer premises.

FIG. 3 show a number of blocks of a communication system, in accordancewith one exemplary embodiment of the present inventive.

FIG. 4 shows a simplified exemplary frame synchronized to the GNSS timeand transmitted by a WiFi transceiver, in accordance with one exemplaryembodiment of the present inventive.

FIG. 5 shows a connectivity device adapted to determine the round-triptime of the packets it transmits, in accordance with one exemplaryembodiment of the present inventive.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention advantageously use the accuratetime reference provided by a wired network, such as the DOCSIS-compliantcable infrastructure. Such accurate timing references are oftenspecified to be synchronized to a GNSS system with atomic clockstability. In the following it is understood that:

-   -   GNSS time refers to the time which is derived from a GNSS-based        time source; the term GNSS time is used herein interchangeably        with absolute time;    -   Geopositioning refers to determining the three-dimensional        coordinates (position) of a device, and optionally its velocity        and time; the term Geopositioning is used herein interchangeably        with the terms positioning or locating;    -   WiFi refers to any wireless connectivity standard, such as        802.11a, 802.11b, 802.11g, 802.11n, Bluetooth, WiMax, Zigbee,        UWB, 60 GHz, and the like;    -   DTI refers to any stable time base provided by a wired system        and which remains synchronous with GNNS time; the DOCSIS Timing        Interface Specification (DTI) is one such time base.

The following embodiments of the present invention are provided withreference to a cable system provisioned with DTI time. It is understood,however, that embodiments of the present invention are equallyapplicable to any other system that provides a stable time base that ismaintained synchronous with the GNSS time with the desired accuracy.

In accordance with one embodiment of the present invention, a GNSSreceiver communicates with any connectivity device, such as a WiFi(wireless LAN), Bluetooth, WiMax and Femtocell device, that is, in turn,in communication with a wired network having access to the DTI. Suchconnectivity devices may set their timing (e.g., frame synchronization)to the DTI and thus serve as Geoposition beacons thus enabling the GNSSreceivers to accurately determine their positions. In accordance withanother embodiment of the present invention, a GNSS receiver uses theDTI supplied by such a network to perform relatively long integrationtime so as to achieve substantially improved sensitivity that isnecessary for indoor Geopositioning applications. Furthermore, the GNSSdata, such as satellite orbital information, may also be propagated bysuch devices at high speed. By providing this data to the GNSS receiversvia such connectivity devices in a rapid fashion, the GNSS receivers areenabled to receive the transmitted data associated with the satellitewithout waiting for the GNSS transmission from the satellites.

Referring to FIG. 2, cable head end 200 which is provisioned with theDTI transmits a cable signal, that is synchronous with the DTI, tocustomer premises (CP), such as CPs 102, 104, 106, and 108. Providingand maintaining precise time synchronization across the cable networkallows reverse (upstream) channels at one such CP to transmit at precisetiming intervals to avoid collisions with other CPs reverse channels. Inother words, for example, CP 102 can transmit upstream to cable head end200 without interfering with upstream signals from CP 104. The distancebetween the cable head end 200 and anyone of the CP's, such as CP 102,may be easily established by, for example, by determining the round-triptime of a signal transmitted from the cable head end 200 to CP 102.

FIG. 3 show a number of blocks of a communication system, in accordancewith one exemplary embodiment of the present invention. Exemplarycustomer premise 300 is shown as including a DOCSIS modem 305 which, inaddition to being compliant with the upstream synchronizationrequirements, provides precise timing to all devices that are incommunication within customer premise 300, shown in this embodiment asincluding a GNSS receiver 310, and a WiFi transceiver 315. The DTI timeprovided by DOCSIS modem 305 remains substantially synchronous, i.e.,within a few nano-second, with the GNSS time.

As described above, DOCSIS modem 305 provides accurate timinginformation to GNSS receiver 310 as well as to WiFi transceiver 315.GNSS receiver 310 uses this timing information as a reference forcorrecting its own timing for integration of the GNSS signal, therebyincreasing its sensitivity to GNSS signals for, e.g., indoor GNSSreception. GNSS receiver 310 may also use the DTI timing information torelatively quickly obtain updated GNSS information such as ephemerisdata for GPS applications; this can be done on a regular basis to ensurethat the integration is carried out over the ephemeris data bits toaccommodate the longer integration times.

Using the DTI timing supplied by DOCSIS modem 305, WiFi transceiver 315is also enabled to synchronize transmission of its preambles and headersin a predetermined fashion to the GNSS time. WiFi transceiver 315 maythus serve as a beacon whose transmissions are used by other receiversto establish their positions. For example, as shown in FIG. 3, thetiming information supplied by WiFi transceiver 315's transmission isreceived by WiFi transceiver 365 via wireless link 350. WiFi transceiver365, in turn, supplies this timing information to GNSS receiver 360.GNSS receiver 360 may use this timing information together with othertiming and position information it receives from either GNSS satellitesor other WiFi transceivers (whose positions are known and aresynchronized to the GNSS time) to determine its position, in any one ofa number ways, as described further below. WiFi transceiver 365 may alsouse the timing information it receives from WiFi transceiver 315 andother WiFi transceivers (all of which are assumed to have knownpositions and are synchronized to the GNSS time) to establish itsposition.

FIG. 4 shows a simplified exemplary frame transmitted by a WiFitransceiver, such as WiFi transceiver 315 that is synchronized to theDTI and the GNSS time. Such a WiFi transceiver, e.g., transceiver 315,is adapted to transmit its preambles such that, for example, thepreamble's header start (HS1) bits are transmitted at, for example,microsecond, 10 microsecond, or 100 microsecond boundaries of theGNSS-time. Therefore, the time of arrival time of the HS1 bits atanother WiFi transceiver, such as WiFi transceiver 365, may be used todetermine the time-of-flight of the signal from WiFi transceiver 315 toWiFi transceiver 365. A WiFi standard, such as 802.11a standard, definesbeacon frames having header bits contain the MAC address of thetransceiver and a body that has the time stamp representing the frame'stransmission time. WiFi transceiver 365 uses the time-of-arrival of theheader bits of the frames transmitted by, e.g., WiFi transceiver 315, asa reference to measure the relative time-of-arrival of the headers bitstransmitted by other WiFi transceivers (not shown). These relative timescan then be used by a trilateration algorithm to establish the locationof WiFi transceiver 365 as well as the absolute time. GNSS receivers 360and 310 may determine their positions using, for example, signals fromfour or more WiFi receivers whose positions are known and whosetransmissions are synchronized with the GNSS time. Alternatively, theGNSS receivers may use, for example, signals from two WiFi receiverswhose positions are known and whose transmissions are synchronized withthe GNSS time, as described above, together with the GNSS signals theyreceive from two satellites to establish their positions. It isunderstood that other combinations of GNSS satellite signals and WiFisignals may be used for position determination by a GNSS receiver whichis not able to receive signals from at least four GNSS receivers touniquely identify its position.

As described above, a WiFi transceiver may also include the time oftransmission information (i.e., time stamp) in the data it transmitswithin a frame, thereby enabling the time-of-flight of a transmissionfrom the transmitting WiFi transceiver to the receiving WiFi transceiverbe established. For example, referring to FIGS. 3 and 4 concurrently,WiFi transceiver 315 may include data which establishes estimated GNSStime associated with, for example, the beginning (i.e. HS1), ending orsome reference point within the frames it transmits. This enables WiFitransceiver 365 to, among other things, determine the time of flightinformation of the transmitted frames if WiFi transceiver 365 has anaccurate estimate of the GNSS time. Furthermore, if WiFi transceiver 365does not know the GNSS time but in addition to receiving such framesfrom WiFi 315, receives similar frames (e.g., frames that include theposition of their associated transmitter as well as the transmissiontime of the frames) from three or more WiFi or other sources, then WiFitransceiver 365 can use standard trilateration techniques to establishits own position and time. If WiFi transceiver 365 receives fewer thanthree such frames (alternatively referred to herein as beacon signals),WiFi transceiver 365 may still be able to estimate its own position withless accuracy, by making assumptions, such as, that it sits at zeroaltitude on the earth's surface. It is understood that any combinationsof the above techniques may be used by a WiFi transceiver to establishits location. A WiFi transceiver may also use the wireless network toobtain updated GNSS system data, such as orbital information, to aid aGNSS receiver, such as GNSS receiver 360 in rapid acquisition ofsatellite signals.

If a WiFi transceiver (e.g., WiFi transceiver 365) receiving thetransmitted frames as described above, has access to the GNSS time, thereceiving WiFi transceiver may directly establish its distance asc·t_(f) from the transmitting WiFi transceiver (e.g., WiFi transceiver315) where c is the speed of light, and t_(f) is the time-of-flight fromthe transmitting WiFi transceiver to the receiving WiFi transceiver. Ifthe position of the transmitting WiFi transceiver is known (e.g. througha database) with a known degree of accuracy, the receiving transceivercan establish its position as being located on a sphere of radiusc·t_(f) with the center of the sphere at the transmitting transceiver. Areceiving transceiver may also be adapted to apply a well-knowntrilateration algorithm to the frames received from other WiFitransceivers, that are also synchronized with the DTI and whosepositions are known, to establish the position of the receivingtransceiver.

A receiving WiFi transceiver, such as WiFi transceiver 365, thatincludes a GNSS receiver, can improve its ability to locate its positionby receiving transmissions, as described above, from the transmittingtransceiver, such as transceiver 315, to either stabilize its timereference or synchronize its time to the GNSS-time. By doing so, thereceiving transceiver can increase the time period over which itperforms coherent integration of the GNSS signals so as to improve itsposition determination. Alternatively, the transmitting transceiver mayaccess a database or an online source which contains the locationinformation of the transmitting transceiver and supply this positioninformation to the receiving transceiver. Such a database may forexample, store the MAC address of a WiFi transceiver and itscorresponding position.

In some embodiments, instead of broadcasting beacons periodically, thetransmitting transceiver may provide GNSS-time and its position onlyupon request by another device. Therefore, the transmitting transceiveracts as a positioning beacon. The receiving device is not required to bein full compliance with, for example, any of the WiFi standards, tobenefit from the beacon information provided by the transmittingtransceiver. The receiving device may be required to implement only suchportions of the standard as are necessary to synchronize with thetransmitting transceiver or to obtain the transmitted position and timeinformation. For example, the receiving device may be required to complyonly with specification defining the receive side, or be able to performsynchronization using only the detected preamble. This enables thereceiving device to be significantly less complicated than other devicesand yet use the beacon information to determine its Geolocation.

FIG. 5 shows a number of components of a connectivity device, such as aWiFi device 500, in accordance with one embodiment of the presentinvention, that is in communication with a WiFi access point 550. WiFidevice 500 is shown as including a module 510 for recording transmissiontime of the packets it transmits by providing the packets with timestamps. The packets are received by access point 550 and sent back toWiFi device 500. Module 520 of WiFi 500 records the receipt time of thepackets transmitted back to the wireless device by access point 550.Accordingly, the round-trip time of a packet transmitted andsubsequently received by WiFi device 500 may be determined. Theround-trip time represents the distance between WiFi device 500 andaccess point 550.

In some embodiments of the present invention, predetermined datasequences may be added to the Geoinfo bits to allow a device to improveits indoor positioning accuracy, i.e., to account for the effects ofmultipath within an indoor environment. For example, in an OFDM system,transmitting a known pattern of data symbols (i.e., referred to as pilottones, training sequences, or preambles) allows the system to accuratelyestimate the channel in a relatively straightforward manner.Accordingly, such training sequences and coding may be added to theGeoinfo data (see FIG. 4) to make transmission of Geoinfo more reliable,thus allowing reliable reception of Geoinfo at much lower signal levelsthan the WiFi standard might otherwise permit.

The above embodiments of the present invention are illustrative and notlimitative. Various alternatives and equivalents are possible. Otheradditions, subtractions or modifications are obvious in view of thepresent invention and are intended to fall within the scope of theappended claim.

What is claimed is:
 1. A method of locating a position, the methodcomprising: receiving, from a wired network, by a customer premisesequipment (CPE) device, timing information that is substantiallysynchronous with a GNSS clock; transmitting the received timinginformation wirelessly by the CPE device; receiving the transmittedusing timing information at a first wireless device; transmitting fromthe timing information by the first wireless device to a second device;and identifying a position of the second wireless device using thetiming information received from the first wireless device andinformation representative of a distance between the second wirelessdevice and a third wireless device.
 2. The method of claim 1 whereinsaid wired network is a DOCSIS-compliant network and the timinginformation is DTI timing information provided by the DOCSIS-compliantnetwork.
 3. The method of claim 1 wherein said wired network is furtheradapted to receive ephemeris information associated with GNSSsatellites.
 4. The method of claim 1 further comprising: receiving thetiming information by a GNSS receiver; and correcting a clock signal ofthe GNSS receiver using the received timing information.
 5. The methodof claim 1 wherein the timing information includes Geolocationinformation representative of a position of the first wireless device.6. The method of claim 5 wherein said Geolocation information includes atime stamp representative of transmission time of the first wirelessdevice.
 7. The method of claim 4 further comprising: receiving at leastone GNSS signal from at least one GNSS satellite by the GNSS receiver;and identifying a position of the GNSS receiver using the receivedtiming information and the at least one GNSS signal.
 8. The method ofclaim 3 further comprising: transmitting the ephemeris informationwirelessly to the first wireless device; and transmitting the ephemerisinformation from the first wireless device to a GNSS receiver.
 9. Themethod of claim 1 further comprising: transmitting the timinginformation from the first wireless device to a GNSS receiver; andenabling the GNSS receiver to use the timing information to determineits position.
 10. The method of claim 1 wherein said first wirelessdevice is a WiFi device.
 11. A method of locating a position of awireless receiver, the method comprising: receiving, from a wirednetwork, by a customer premises equipment (CPE) device, timinginformation that is substantially synchronous with a GNSS clock;transmitting the timing information wirelessly by the CPE device to thewireless receiver; and locating the position of the wireless receiverusing the transmitted timing information.
 12. The method of claim 11wherein said wireless receiver is a GNSS receiver.
 13. The method ofclaim 11 wherein said wireless receiver is a WiFi receiver.
 14. Themethod of claim 11 wherein said wired network is a DOCSIS-compliantnetwork and the timing information is DTI timing information provided bythe DOCSIS-compliant network.
 15. The method of claim 11 wherein the CPEdevice comprises: a modem configured to couple to the wired network; anda wireless transceiver coupled to the modem and configured to transmitthe timing information.
 16. The method of claim 15 wherein the CPEdevice further comprises a GNSS receiver coupled to the modem and thewireless transceiver.
 17. The method of claim 16 wherein the GNSSreceiver increases its sensitivity by integrating the received timinginformation over a period of time.
 18. The method of claim 11 whereinthe timing information comprises Geolocation information representativeof a position of the CPE device.
 19. The method of claim 18 wherein theGeolocation information comprises a time stamp representative oftransmission time of the CPE device.